This invention relates to an improved process for preparation of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine otherwise known as atrazine herbicide. More particularly, this invention is directed to an improvement on the conventional process for preparing atrazine by stepwise replacement of chlorine atoms on cyanuric chloride in a mixed water/acetone solvent system in which the purity of the desired 2-chloro-4-ethylamino 6-isopropylamino-4-triazine is held at a high level by minimizing the formation of 2-chloro-4,6-bis(alkylamino)-s-triazine impurities in the process.
2-Chloro-4-ethylamino-6-isopropylamino-s-triazine is well known in the agricultural field, under the trade name atrazine, as a selective herbicide for weed control among cultivated crops, especially corn. Although other synthetic routes are possible, much, if not all, of the commercial scale atrazine production is based on the stepwise replacement of chlorine atoms on cyanuric chloride with the appropriate alkyl amines in the presence of an acid acceptor, typically an alkali metal hydroxide. In fact, the patent art is replete with a variety of disclosures or suggestions of ways to modify and/or optimize this basic reaction scheme to obtain alkyl amino substituted s-triazine herbicides such as atrazine in high yield and purity e.g. see U.S. Pat. Nos. 3,376,302; 3,436,394; 3,586,679; 3,590,040; 3,639,399 and 3,883,515. According to these previous patent disclosures, a variety of solvent systems ranging from mixed aqueous/organic to anhydrous organic solvents can be suitably used in preparing the desired substituted triazine herbicide from cyanuric chloride. A particularly advantageous solvent system for the stepwise reaction scheme, at least from the standpoint of availability and handling ease, is a mixed acetone and water solvent system such as is disclosed in U.S. Pat. No. 2,891,855.
While the state of the art on the synthesis of substituted alkylamino-s-triazine herbicides from cyanuric chloride is rather advanced, as is evidenced by the above-mentioned patent disclosures, commercial scale production units for such triazine herbicides are not devoid of problems. One source of continuing problems is the occurrence of side reactions in the triazine synthesis which lead to alkylamino substituted s-triazine by-products in the desired triazine product. These triazine by-products must generally be held to rather low levels in the final product to assure consistent biological performance and handling properties in the field. In this regard, at least one type of substituted triazine by-product i.e. tris(alkylamino)-s-triazine, has received considerable attention, with several techniques being suggested e.g., U.S. Pat. Nos. 3,681,335; 3,681,337 and 3,705,156, to avoid or suppress its formation. In the case of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine or atrazine, there are at least two other alkylamino substituted s-triazine by-products which cause considerable concern. These two by-products i.e., 2-chloro-4,6-bis(ethylamino)-s-triazine and 2-chloro-4,6-bis(isopropylamino)-s-triazine are themselves active herbicides and, as a result, it is generally necessary to restrict the quantity of each to a very low level in the final product to avoid possible performance problems in practical use situations. Typically, the specification for technical atrazine requires that neither one of these impurities exceed one percent by weight of the technical product. As a general matter, this impurity specification can be met by carefully controlling the molar charge ratio of cyanauric chloride and alkylamine i.e. ethylamine and isopropylamine, reactants at, or very close to, the stoichiometric ratio for the two reactions i.e. 1 to 1 mole ratio of alkylamine to cyanuric chloride or dichloro intermediate. While this measure of control is possible on the laboratory scale and with certain solvent systems and reaction techniques on a larger scale, it is very difficult to obtain adequate control and mixing on a commercial scale, especially with the mixed acetone/water solvent system, to insure that specification atrazine can be consistently produced. The use of a small molar excess of isopropylamine in the first stage of the synthesis invariably leads to the formation of excessive amounts of 2-chloro-4,6-bis(isopropylamino)-s-triazine whereas charging less than the stoichiometric amount of isopropylamine gives rise to the formation of excessive quantities of 2-chloro-4,6-bis(ethylamino)-s-triazine in the second reaction stage. Deviations as small as 3-5% from the stoichiometric ratio in the first reaction stage can lead to the formation of excessive amounts of either of these impurities depending on the reaction conditions and technique selected.
From the foregoing, it is apparent that it would be extremely advantageous if a technique could be developed whereby the formation of 2-chloro-4,6-bis(ethylamino)-and-4,6-bis(isopropylamino)-s-triazine could be minimized in the synthesis of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine from cyanuric chloride without requiring that the molar charge ratio of cyanuric chloride to alkylamine reactant be maintained within impractically narrow limits.